(1) Field of the Invention
The present invention relates to a developing device and an image forming apparatus including the developing device, and in particular to a technology for stabilizing the amount of developer in a developing tank in a developing device of a trickle system using a two-component developer.
(2) Description of Related Art
In recent years, developing devices of a so-called trickle developing system have become popular, among developing devices of a two-component developing system in which a developer composed of toner and carrier is used. In the developing devices of the trickle developing system, as disclosed in Japanese Patent Application Publication No. 2005-221852 for example, while a part of the developer is kept to be ejected to outside of a developing tank from an outlet, a fresh developer is replenished into the developing tank so that a carrier with a decreased charging capability is replaced with a fresh carrier.
FIG. 8 is a schematic cross sectional view illustrating a conventional structure of a developing device 500 adopting such a trickle developing system.
As shown in FIG. 8, the developing device 500 includes a developing roller 531, a conveying screw 532, a stirring screw 533, and a housing 534 housing these.
The inner space of the housing 534 is partitioned by a partition wall 534e into a first conveying passage 534a and a second conveying passage 534b, with a first communication path 536a and a second communication path 536b being formed for communications between the first conveying passage 534a and the second conveying passage 534b. These passages and paths form a circulating passage 534f in which the developer is circulated.
Also, a developer ejecting passage 534c is formed as an extension of the first conveying passage 534a on the downstream side in the developer conveying direction, and a developer supply passage 534d is formed as an extension of the second conveying passage 534b on the upstream side in the developer conveying direction (hereinafter, the upstream side and the downstream side in the developer conveying direction in each conveying passage may be merely referred to as “upstream side” and “downstream side” in each conveying passage, respectively).
The developing roller 531 is formed along the first conveying passage 534a. 
The conveying screw 532 and the stirring screw 533 are spiral screws. Among these, the conveying screw 532 is installed in the first conveying passage 534a to convey the developer therein toward the first communication path 536a. 
Furthermore, a reverse winding screw 532c is provided at a position corresponding to the first communication path 536a on the downstream side of the first conveying passage 534a. The reverse winding screw 532c is a screw sharing a rotation shaft with the conveying screw 532, and the vanes thereof are attached to the rotation shaft at a reverse angle of an angle at which the vanes of the conveying screw 532 are attached.
With the above structure, a small amount of developer (toner and carrier) continues to be supplied from a toner replenishing device (not illustrated) into the housing 534 via an inlet hole 538. The developer is conveyed through the developer supply passage 534d to the second conveying passage 534b, and there it is mixed by an operation of the stirring screw 533 with a developer conveyed from the first conveying passage 534a via the first communication path 536a, and then the developer is conveyed to the first conveying passage 534a via the second communication path 536b. 
In this circulation, the developer reaches the reverse winding screw 532c on the downstream side of the first conveying passage 534a. However, when a small amount of developer is present in the first conveying passage 534a, the developer having reached the reverse winding screw 532c is pushed back in the reverse direction. As a result, most of the developer is conveyed to the second conveying passage 534b through the first communication path 536a. 
On the other hand, when a large amount of developer is present in the first conveying passage 534a, although the developer having reached the reverse winding screw 532c receives a force pushing back the developer in the reverse direction, the pressure of the developer congesting in a region G in front of the reverse winding screw 532c is high, and the developer overcomes the pushing back force of the reverse winding screw 532c and passes the reverse winding screw 532c, and is ejected into a collection box (not illustrated) via an outlet hole 537. With this structure, the amount of developer in the housing 534 is kept to be stable.
However, in some types of electrophotographic image forming apparatuses, the speed of the entire process (system speed) varies depending on the type of paper or due to the image quality driving. In such types of electrophotographic image forming apparatuses, when the system speed changes, the speed at which the developer circulates in the developing device 500 changes as well. This causes the pressure and/or bulk of the developer to change partially.
In a developing device with the above structure illustrated in FIG. 8, the amount of ejected developer basically depends on the pressure of developer in the region G on the downstream side in the first conveying passage 534a. That is to say, it receives directly the influence of change of pressure due to the change of the system speed. Accordingly, it is difficult for the developing device to keep the amount of developer constant.
For example, when driven at a high speed, both of the following increase: (i) an amount of developer conveyed per unit time by the conveying screw 532 in the forward direction (hereinafter, the amount is referred to as “conveying power”); and (ii) an amount of developer conveyed per unit time by the reverse winding screw 532c in the reverse direction. Due to this, the pressure of the developer in the region G increases as well. Normally, the conveying power in the forward direction is set to be larger than that in the reverse direction. In that case, there is a possibility that, when the device is driven at a high speed, the conveying power in the forward direction increases more than the conveying power in the reverse direction, the difference between the conveying powers in the forward and reverse directions becomes large, the pressure of the developer increases, an excessive amount of developer passes the reverse winding screw 532c to be ejected, and a more amount of developer is ejected than when the device is driven at a low speed.
In particular, in order to respond to the demand for miniaturization of image forming apparatuses in recent years, the developing device itself is required to be smaller. Accordingly, the capacity of the developing tank in the developing device has to be smaller as well. In that case, a slight difference in the amount of ejected developer may greatly change the height of the liquid surface of the developer contained in the supply conveying passage facing the developing roller, and if the height of the developer decreases temporarily due to ejection of an excessive amount of developer, a sufficient amount of developer may not be supplied to the developing roller, and an uneven image may occur.